1. Field of the Invention
The invention relates to a process for the production of articles composed of aluminum or magnesium alloys through the use of thermoforming techniques, such techniques including hot plastic deformation of thin-walled blanks by means of a fluid under pressure, the pressure causing the blank to be applied to the surface of a mold.
2. Description of the Prior Art
Thermoforming, a very widely used process in the plastics material industry, comprises the raising of a thin-walled blank, in most cases a cup-shaped member or a simple flat sheet, to an elevated temperature which is lower than the melting temperature of the material being used but which is sufficient to soften that material and to provide it with good plasticity. The desired shape is then imparted to the blank by applying the blank against the surface of a mold by the action of a fluid under pressure. In regard to blanks whose walls are sufficiently malleable at the forming temperature, it is also possible to use simple atmospheric pressure as the forming pressure by creating a vacuum between the blank and the forming surface of the mold.
In recent years, the use of this thermoforming method has been extended to the production of many thin-walled articles formed of special aluminum alloys, these alloys being referred to as "superplastic alloys". Many patents disclose superplastic aluminum alloy compositions and various alternative ways of performing the thermoforming process, among them, French Pat. Nos. 2,004,410, 2,146,847 and 2,245,428.
In such methods of forming thin-walled articles, the periphery of the metal blank is held in place by being gripped between the edges of a two-part mold, the periphery of the blank not being deformed. This gripping action seals the space between the forming surfaces of the mold and the facing surface of the blank with respect to ambient. Only the portion of the blank which is disposed facing the recessed female portion (or projecting male portion) of the mold undergoes plastic deformation by elongation of the metal wall in every direction, no slippage occurring between the periphery of the blank and the edges of the two-part mold gripping said periphery. When plastic materials such as organic plastics are subjected to thermoforming, the forming of the article may be effected either by the action of a fluid under pressure, which pressure is applied to the face of the blank which is to be deformed to provide a hollow configuration, or by forming a vacuum on the face which is to be deformed to provide a projecting relief configuration. However, if it is necessary to form an article from a sheet of metal having a thickness too great to be affected by the differential pressure obtainable by the inducing vacuum and the atmosphere, a relatively substantial fluid pressure must be used to apply the wall of the blank against the surface of the mold.
Thermoforming is typically referred to as positive thermoforming when the mold used is a relief mold, such a mold being, except for contraction of the metal after cooling, of the dimensions of the interior of the article to be produced. When the mold used is a hollow mold which, except for shrinkage, is of the dimensions of the outside of the article to be produced, the process so involved is referred to as negative thermoforming. Through the use of these thermoforming techniques, "superplastic" alloys, as referred to hereinabove, can be subjected to substantial degrees of deformation without rupture, particularly degrees of elongation on the order of 1000 to 2000% at temperatures which are from 0.3 Tf to 0.6 Tf, Tf being the aboslute melting temperature of the alloy being used as the blank. Such alloys make it possible to produce articles whose developed surface area S.sub.1 is from 3 to 4 times the surface area S.sub.0 of the starting blank. However, deformation of the blank must be slow and requires from 4 to 10 minutes per operation. Thus, this method is not suitable for high-speed production of mass-production products.
According to the present invention, it is demonstrated in contrast to conventional practices in the art and contrary to generally accepted ideas, that a thermoforming process can be used with blanks of current aluminum alloys such as alloys 2002, 3003, 4047, 7020, 8011 and 5754, in accordance with French Standard A02 104. The present invention has also been found to be useful in the formation of thermoformed articles from blanks composed of magnesium alloys.